DESCRIPTION This is a renewal proposal for a comparative study of cytoplasmic and organelle seryl tRNA synthetases (SerRS) from yeast and plants, Arabidopsis and maize. Specific aims include the cloning, sequencing and expression of the mitochondrial seryl tRNA synthetase from S. cerevisiae, the chloroplast SerRS from maize and the cytoplasmic SerRS from Arabidopsis. Substrate binding domains of the various isozymes will be analyzed by site-directed mutagenesis. An additional goal is the identification of the nucleotides in seryl-tRNA's which are responsible for recognition by their cognate synthetases.